meson: drop libdl, threads, and librt dependencies

Our baseline glibc is 2.34, which merged libdl, libpthread (the
dependency('threads') target), and librt into libc. Empty .so/.a stubs
remain for backward compatibility with old binaries, but new builds
resolve dl_*, pthread_*, mq_*, timer_*, etc. directly from libc.
On musl the same libraries are likewise empty stubs.

Drop the libdl, threads, and librt entries from every meson.build, and
remove the now-stale 'Libs.private: -lrt -pthread' from libudev.pc.in
since both flags resolve to empty link-time stubs on glibc 2.34+ and
musl.

Verified with readelf -d that libsystemd.so, libudev.so, and systemd no
longer carry DT_NEEDED entries for libdl/libpthread/librt.

src/shared/meson.build

@@ -383,8 +383,7 @@ shared_sources += [dns_type_from_name_inc, dns_type_to_name_inc]
 
 libshared_name = 'systemd-shared-@0@'.format(shared_lib_tag)
 
-libshared_deps = [threads,
-                  libacl_cflags,
+libshared_deps = [libacl_cflags,
                   libapparmor_cflags,
                   libarchive_cflags,
                   libaudit_cflags,
@@ -393,7 +392,6 @@ libshared_deps = [threads,
                   libcrypt_cflags,
                   libcryptsetup_cflags,
                   libcurl_cflags,
-                  libdl,
                   libdw_cflags,
                   libelf_cflags,
                   libfdisk_cflags,
@@ -410,7 +408,6 @@ libshared_deps = [threads,
                   libpcre2_cflags,
                   libpwquality_cflags,
                   libqrencode_cflags,
-                  librt,
                   libseccomp_cflags,
                   libselinux_cflags,
                   libxenctrl_cflags,